1. Field of the Invention
The invention relates generally to a device for percutaneously implanting an imaging marker for identifying a location within a tissue mass. More particularly, the invention relates to a device for implanting a subcutaneous imaging marker that comprises at least two elements, each of which have a primary imaging mode.
2. Description of the Related Art
Subcutaneous imaging markers are commonly implanted to identify a particular location in various areas and organs of the body. For example, markers are positioned at biopsy sites so that a practitioner can readily identify the tissue sample location after the biopsy procedure is completed. Markers are also used to denote the locations of lesions for therapeutic procedures, such as chemotherapy.
Once the marker is implanted, it can be viewed using several well-known medical imaging techniques, such as radiography, ultrasonography, and magnetic resonance imaging (MRI). In radiography, x-rays, which are wavelike forms of electromagnetic energy carried by particles called photons, passed through the body are either scattered, absorbed, or transmitted by the hard and soft tissues. Hard tissues are more likely to absorb the x-ray photons, while the soft tissues tend to transmit the x-ray photons. The transmitted photons are recorded by a detector, such as an x-ray photographic film or a digital receiver, which produces a two-dimensional negative film image. Consequently, bones and other hard tissues appear white in the image, and organs, muscles, and other soft tissues appear black or gray. Mammography is a form of radiography where low dose x-ray photos are passed through a breast under compression to diagnose breast disease in women. In computerized axial tomography (CAT), another form of radiography, the x-ray source and the x-ray detectors revolve around the body, or the source remains stationary, and the x-ray beam is bounced off a revolving reflector. A machine records x-ray slices across the body in a spiral motion. After the patient passes through the machine, the computer combines all the information from each scan to form a three-dimensional detailed image of the body.
Ultrasonography involves emitting a beam of high frequency, about 3-10 MHz, pulses of acoustic energy from a transmitter and onto body tissue surfaces oriented perpendicular to the transmitter. Some of the acoustic energy pulses reflect at boundaries between tissues having a difference in acoustic impedance, which is a medium's resistance to transmission of acoustic energy, and the echo is detected by an acoustic transducer, which transforms the echo into an electrical pulse. Some of the energy transmits past the boundary until it reaches another boundary where it can reflect back to the transducer. The electric pulse is sent to a computer with a display, and the computer forms a two-dimensional image by determining the proper location of a dot, and its corresponding shade of gray, on the display screen. As the difference in acoustic impedance at a boundary increases, more sound energy is reflected. Body tissue has an acoustical impedance over 3000 times that of air; consequently, entrapped air can be used in subcutaneous imaging markers in order to enhance the visibility of the marker during ultrasonography. Additionally, the texture of the marker can increase the scattering of the acoustical energy pulses.
In MRI, the patient is positioned inside a strong magnetic field usually generated by a large bore superconducting magnet. Specifically, the body part to be scanned is placed in the exact center or isocenter of the magnetic field, and the MRI scanner takes several slices that can be combined to form two-dimensional images or three-dimensional models. Markers comprising non-magnetic materials are viewable with MRI.
Generally speaking, markers have several imaging modes where they can be viewed with any of the above imaging techniques; however, each marker has a primary imaging mode wherein the marker is best viewed or most easily distinguished. For example, a metal clip having a simple, thin shape can be difficult to discern with ultrasonography if the marker is oriented on its side relative to the acoustic emitter. On the display, which is typically grainy, the marker will appear as a very thin, undistinguishable line. On the other hand, such a marker is readily seen with x-ray, regardless of its orientation, because of the sharp contrast in x-ray transmission between the metal and the surrounding soft tissue. Accordingly, the metal marker has an ultrasound imaging mode and an x-ray imaging mode, and the x-ray imaging mode is the primary imaging mode. Other markers, such as those with entrapped air, can be seen easily with ultrasonography but are not as visible in an x-ray imaging mode because they transmit the x-ray photons in a manner similar to the soft tissue. Such markers also have an ultrasound imaging mode and an x-ray imaging mode, but the primary imaging mode is the ultrasound imaging mode. In selecting a marker, a practitioner is most likely to choose a marker that has a primary imaging mode corresponding to a preferred imaging technique. However, such a selection can preclude the effective use of other imaging techniques. For example, in some procedures the marker is permanent and will be imaged multiple times by different technicians over a relatively long time span, possibly over several years. During that time, different imaging techniques might be used. Thus, it is desirable for a marker to have multiple primary modes.